The present invention relates to an apparatus and method of an adaptive weighted parallel interference cancellation system for CDMA. More particularly, the present invention relates to an apparatus and method of a adaptive weighted parallel interference cancellation system for CDMA, which decreases multiple access interference (MAI) and thereby increases general performance.
Direct-sequence code-division multiple access (DS-CDMA) technology is the most attractive and promising candidate for the next generation wireless communication systems such as IMT-2000 and UMTS. The performance and/or the capacity of conventional CDMA detectors are greatly influenced by the MAI contributed by the other users. Even though optimal multi-user detection is not interference-limited, it is too complex to be implemented. Thus, a compromise between performance and system complexity gives a birth to sub-optimal multi-user detection like the interference cancellation (IC), including successive IC (SIC) and parallel IC (PIC). To achieve the acceptable performance improvement with the conventional PIC (CPIC), in which the total amount of the MAI estimate is cancelled at each stage of iteration, accurate channel estimation and bit decision are required. However, they are not guaranteed in low signal to noise and interference ratio (SNIR) situations.
To overcome the drawback of the CPIC, a few modified CPIC schemes were proposed based on similar idea. One is the adaptive hybrid serial/parallel IC (AHSPIC), which is devised for the (multi-path) fading environments. The basic idea of the AHSPIC is that it keeps the detectors with low SNIR input signals from participating in cancellation. That is, the detectors not involved in earlier cancellations are to be included in later cancellations when sufficient SINR is guaranteed after canceling the signals with high power in earlier cancellations.
The Partial PIC (PPIC) is another approach designed with focusing on the case of fixed channels with equal power. In the PPIC the partial amount of MAI estimate is cancelled at each stage of iteration. As the IC operation progresses, the fidelity of estimates for regeneration of the MAI goes up and thus the weight determining the amount of the MAI estimate being cancelled increases. The tentative soft decision of the PPIC at the present stage is obtained as a weighted sum of soft tentative decision available at the previous stage and the received signal from which MAI estimate is cancelled out. As a result, the performance improvement is not good enough and the whole system gets complicated.
The present invention provides an apparatus and method of an adaptive weighted parallel interference cancellation system for CDMA that obtains MAI-reduced signals using adaptive weighted subtraction of interference.
An apparatus of an adaptive weighted parallel interference cancellation system for CDMA comprises K stages for iterative cancellation of MAI in received signal. Each stage comprises a detector, a regenerator, and a subtractor. The detector determines tentative bit decision from the despread signal of a user. The regenerator that is coupled to the detector in parallel spreads an unmodulated signal, multiplies a channel phase, and multiplies by the output of the detector. The subtractor obtains MAI-cancelled signal on the basis of the adaptive weight adjustment.
Desirably, the detector comprises a matched filter, a real-part selector, a gain controller, and a finite-bit quantizer. The matched filter obtains maximum SNR (signal to noise ratio) from the despread signal of a user. The real-part selector extracts a real-part from the despread signal of a user with maximum SNR. The gain controller adjusts the gain depending upon the fidelity of the output signal of the real-part selector. The finite-bit quantizer quantizes the gain-adjusted signal from the gain controller. The finite-bit bit quantizer performs the hard decision for the signal larger than one and the soft decision for the signal less than one.
Desirably, the regenerator comprises a spreading code multiplier, a pulse shaping filter, a channel phase multiplier, and a multiplier. The spreading code multiplier multiplies the unmodulated signal by the spreading code of a user. The pulse shaping filter shapes the output signal of the spreading code multiplier. The channel phase multiplier multiplies the channel phase of a user by the output signal of the pulse shaping filter. The multiplier multiplies the output signal of the channel phase multiplier by the output signal of the detector.
Desirably, the subtractor comprises a weight multiplier, a first adder, a subtractor, and a second adder. The weight multiplier multiplies the output signal of the multiplier by the weight of the present stage. The first adder sums weight multiplied signals of all users. The subtractor subtracts the output signal of the first adder from the received baseband signal. The second adder adds the output signal of the weight multiplier to the output signal of the subtractor and thereby MAI-cancelled signal of a user is obtained.
The subtractor further comprises a first multiplier and a second multiplier. The first multiplier multiplies the output signal of the second adder by spreading code of a user. The second multiplier multiplies the output signal of the first multiplier by the channel phase and thereby phase difference is compensated.
A method of an adaptive weighted parallel interference cancellation system for CDMA comprises the following:
a) Tentative bit decision is made on the basis of the despread signal of a user.
b) The unmodulated signal is spread and a channel phase is multiplied.
c) The tentatively decided bit is multiplied by the spread and phase-multiplied signal and thereby the transmitted signal of a user is generated.
d) The received signal is multiplied by the weight and MAI-cancelled signal of a user is generated.
e) The above a), b), c), and d) steps for canceling MAI.
Desirably, the tentative bit decision comprises the following: First, maximum SNR from the despread signal of a user is obtained. Second, real parts from the despread signal having maximum SNR are extracted. Third, the gain of extracted real parts from the despread signal is controlled. Fourth, tentative bit decision of the gain-controlled signal is made.
Desirably, b) and c) from above comprises as follows: First, multiply unmodulated signal by spreading code of a user. Second, shape the output signal of the previous step. Third, multiply the shaped output signal of the previous step by channel phase of a user. Fourth, multiply the shaped output signal of the second step by the tentatively decided bit from a) above. Desirably, d) from above comprises as follows: First, multiply the received signal of a user by the weight of the present stage. Second, sum the weight multiplied signals of all users. Third, subtract the summed signals from the received baseband signal. Fourth, add the output signal of the first step to the output signal of the third step and an MAI-cancelled signal is obtained. Desirably, the third step comprises the following two steps. First is to multiply the MAI-cancelled signal of a user by the spreading code of a user. Next is to multiply the output signal of the first step by the channel phase and thereby the phase of the channels reflected.